Replaced dynamic module attributes with actual references

My IDE would complain about unresolved references when working with the httpretty package.
To fix this I replaced the dynamic attributes with references to the httpretty classmethods and attributes.
I don't know what the reason was to create those attributes dynamically during runtime in the first place, though.

Running tox passes all tests for py26 and py27 (py33 tests seem to be broken for other reasons).